Poly(arylene sulfides) (hereinafter abbreviated as “PASs”) represented by poly(phenylene sulfide) (hereinafter abbreviated as “PPS”) are engineering plastics excellent in heat resistance, chemical resistance, flame retardancy, mechanical strength, electrical properties, dimensional stability, etc. The PASs are commonly used in a wide variety of fields such as electrical and electronic equipments and automotive equipments because they can be molded or formed into various kinds of molded or formed products, films, sheets, fibers, etc. by general melt processing processes such as extrusion, injection molding and compression molding.
As a typical production process of a PAS, is known a process in which a sulfur source is reacted with a dihalo-aromatic compound in an organic amide solvent such as N-methyl-2-pyrrolidone. As the sulfur source, is generally used an alkali metal sulfide, an alkali metal hydrosulfide or a mixture thereof. When the alkali metal hydro sulfide is used as the sulfur source, the alkali metal hydrosulfide is used in combination with an alkali metal hydroxide. When at least one sulfur source selected from the group consisting of alkali metal sulfides and alkali metal hydrosulfides is reacted with a dihalo-aromatic compound, a great amount of an alkali metal salt such as NaCl is secondarily produced by a desalting condensation reaction.
For example, Japanese Patent Publication No. 45-3368 (Patent Literature 1) discloses a process for producing poly(phenylene sulfide) by reacting sodium sulfide and p-dichlorobenzene in an organic amide solvent such as N-methyl-2-pyrrolidone. However, the process disclosed in Patent Literature 1 can provide only PPS having a low molecular weight. When the low-molecular weight PPS is heated in the presence of air to cure it, the molecular weight thereof can be increased. However, the PPS whose molecular weight has been increased by the oxidative cure (curing) is insufficient in mechanical properties such as toughness.
Thus, there have been proposed various processes for producing a high-molecular weight PAS using various polymerization aids in a production process of a PAS by polymerizing a sulfur source and a dihalo-aromatic compound in an organic amide solvent. For example, Japanese Patent Publication No. 52-12240 (Patent Literature 2) discloses a production process of PPS using a carboxylic acid alkali metal salt as the polymerization aid. Japanese Patent Application Laid-Open No. 59-219332 (Patent Literature 3) discloses a production process of PPS using an alkaline earth metal salt or zinc salt of an aromatic carboxylic acid as the polymerization aid. U.S. Pat. No. 4,038,263 (Patent Literature 4) discloses a production process of PPS using an alkali metal halide as the polymerization aid. Japanese Patent Application Laid-Open No. 1-161022 (Patent Literature 5) discloses a production process of PPS using the sodium salt of an aliphatic carboxylic acid as the polymerization aid. Japanese Patent Publication No. 63-33775 (Patent Literature 6) and Japanese Patent Application Laid-Open No. 8-183858 (Patent Literature 7) disclose a production process of a PAS using water as the polymerization aid.
The amount and time of the polymerization aid added, the temperature of a polymerization reaction system, etc. are controlled, whereby a phase-separated state that a concentrated formed polymer phase and a dilute formed polymer phase are present in a mixed state can be created in a liquid phase within the polymerization reaction system. When the polymerization reaction is continued in such a phase-separated state, the formation of a high-molecular weight PAS is caused to progress, and moreover the high-molecular weight PAS can be obtained in the form of granules by slowly cooling the polymerization reaction system after the polymerization reaction. Therefore, these polymerization aids are called phase separation agents.
More specifically, when at least one sulfur source selected from the group consisting of alkali metal sulfides and alkali metal hydrosulfides is reacted with a dihalo-aromatic compound, a desalting condensation reaction between the monomers is caused to rapidly progress to increase the conversion of the dihalo-aromatic compound. However, the polymer in this state is low in both melt viscosity and molecular weight and is in a stage of the so-called prepolymer.
When a phase separation agent is caused to exist in a liquid phase within a high-temperature polymerization reaction system, a phase-separated state that a concentrated formed polymer phase and a dilute formed polymer phase are present in a mixed state in the liquid phase is created. An alkali metal salt such as NaCl is present in a great amount in the dilute formed polymer phase. The concentrated formed polymer phase contains many of a formed polymer in the liquid phase. When the liquid phase is stirred, the concentrated formed polymer phase is dispersed in the dilute formed polymer phase, and a condensation reaction between the prepolymers is caused to efficiently progress in the concentrated phase. As a result, the formation of a high-molecular weight polymer is advanced.
When the liquid phase is sifted by a screen after the polymerization reaction, a granular PAS having a high molecular weight is captured on the screen. Impurities such as a secondarily formed alkali metal salt and oligomers are easy to be removed by washing from the high-molecular weight granular PAS. At present, the high-molecular weight granular PAS is washed by a combination of water washing, washing with an organic solvent, acid washing, etc., in a post treatment step after polymerization whereby a PAS substantially containing no alkali metal salt such as NaCl comes to be obtained.
As described above, the high-molecular weight granular PAS can be obtained according to the production process of the PAS comprising the phase-separation polymerization step using the phase separation agent. Impurities such as the secondarily formed alkali metal salt and oligomers are easy to be removed by washing from the granular PAS. On the contrary, the yield of the granular PAS is not always sufficiently high, and a great amount of oligomers or the like removed by washing is disposed as industrial waste.
With the increasing interest in environmental problems in recent years, a production process little in industrial waste has been more and more required even in a technical field of polymers. Even in the production process of a granular PAS comprising the phase-separation polymerization step, there is a latent requirement for reduction in waste such as oligomers and a finely particulate PAS. When the yield of the granular PAS is improved, it is expected to reduce the production of the oligomers and finely particulate PAS.
However, interest in reduction of the production of the oligomers and finely particulate PAS has been little in the prior art, and moreover there has been a limit to technical solving means for increasing the yield of the granular PAS. In addition, in the production process of the PAS comprising a step of polymerizing the sulfur source and the dihalo-aromatic compound in the organic amide solvent, it has been proved that it is preferable to adopt a process of adding an alkali metal hydroxide (for example, NaOH) in excess to the charged sulfur source within the polymerization reaction system for the purpose of stabilizing the polymerization reaction. According to this process, however, there is a tendency for the production of pasty oligomers and a finely particulate PAS to increase, and the yield of the granular PAS is lowered. This tendency becomes stronger as the molecular weight or melt viscosity of the PAS is higher.